Notifications that are generated in the context of a web browser tend to be modal notifications in the form of a dialog that blocks the user from their browsing task. Specifically, notifications regarding downloads tend to be presented in the primary field of view, overlay and block content and, because of their modal nature, force a user to take some action before being able to return to their browsing task.
These types of notifications can be distracting and annoying for many users. Moreover, requiring a user to take an action before allowing them to return to the browsing task can degrade the user experience.